3Com S7906E Configuration Manual page 650

A Type-3 LSA can be used to advertise a default route, having the Link State ID and Network Mask set
to 0.0.0.0.
4) AS external LSA
An AS external LSA originates from an ASBR, describing routing information to a destination outside
the AS.
Figure 1-19 AS external LSA format
Major fields:
Link State ID: The IP address of another AS to be advertised. When describing a default route, the
Link State ID is always set to Default Destination (0.0.0.0) and the Network Mask is set to 0.0.0.0
Network mask: The IP address mask for the advertised destination
E (External Metric): The type of the external metric value, which is set to 1 for type 2 external routes,
and set to 0 for type 1 external routes. Refer to
types
Metric: The metric to the destination
Forwarding Address: Data traffic for the advertised destination will be forwarded to this address
External Route Tag: A tag attached to each external route. This is not used by the OSPF protocol
itself. It may be used to manage external routes.
5) NSSA external LSA
An NSSA external LSA originates from the ASBR in a NSSA and is flooded in the NSSA area only. It has
the same format as the AS external LSA.
Route types for description about external route
1-17